“What the hell is a Republican doing in San Francisco?” he blared to a crowd of 150 Bay Area tech entrepreneurs intrigued enough to come hear him speak on Saturday.

The people who support his crusade against the National Security Agency’s surveillance dragnet? “The leave-me-the-hell-alone coalition,” Mr. Paul calls them.

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An "NSA Spy Cam Blocker" is available for sale on Rand Paul's website.

Then there is his biggest and most reliable stump-speech applause line: “What you do on your phone is none of the government’s damn business.” Mr. Paul may soon put it on T-shirts. They would make nice companions to the $15 “NSA Spy Cam Blocker,” which covers laptop webcam lenses in hopes of shutting Big Brother’s prying eyes.

As Congress and the courts try to resolve the difficult question of how far the federal government may go in collecting data on citizens’ private communications, Mr. Paul has become the most unabashed and unambiguous opponent of renewing those far-reaching powers of any presidential candidate.

“The right to be left alone is the most cherished of rights,” he said here over the weekend. “There’s not one other candidate on the Republican side or the Democratic side who’s willing to say: ‘On Day 1 I’d stop it all. I would end all bulk collection of records.’ ”

Few of Mr. Paul’s stands arouse greater suspicion from conservatives, who are eager to caricature him as a peacenik and a kook. And few issues make him stand out so starkly against a field of rivals who are largely in agreement that the intelligence agencies should have broad, unencumbered powers to pursue terrorists.

Yet as Congress debates extending the Patriot Act beyond its June 1 expiration, Mr. Paul, of Kentucky, is determined to seize the spotlight and test whether a candidate so vocal about safeguarding civil liberties can succeed in a party that increasingly demands fealty to hawkish dogma on national security and defense.

In an interview, Mr. Paul said he would use the next few weeks to deliver floor speeches and introduce a series of amendments aimed at curtailing government surveillance. (On Monday in New Hampshire, he told The Union Leader he would lead a filibuster against renewal of the Patriot Act.)

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Senator Rand Paul spoke about surveillance programs on Saturday in San Francisco.CreditJason Henry for The New York Times

“Someone has got to defend the Constitution,” he said in the interview on Saturday.

A spirited legislative battle on government surveillance could also energize Mr. Paul’s core of libertarian supporters, who have detected a reticence lately in his attitude toward the ideals that helped fuel his political rise. He signed onto a confrontational letter that his Senate colleagues sent to Iran’s leaders, warning against cutting a deal with President Obama over their nuclear program. He also appeared to mock Baltimore protesters after the death of Freddie Gray, despite having advocated in the past for a crackdown on the use of excessive force by the police.

Mr. Paul acknowledges that sometimes his mouth gets him in trouble. But he said no one should have any reason to doubt his beliefs. “I talk too much,” he said. “I will say things that aren’t perfect. You don’t get this from Hillary Clinton because she won’t talk to you.”

His call to let the Patriot Act expire, however, is no offhand remark.

The issue is one that animates him. And it could help put him back at the center of the debate over national security, a topic he has ceded lately to rivals who appear much more comfortable taking a hard line against hostile regimes like Iran.

“You can be a minority because of the color of your skin or the shade of your ideology,” he said on Saturday. “So there’s every reason in the world we have to be concerned about a government that collects all of our records all of the time.”

Polls show that Republicans generally approve of the bulk records collection program; many even want it strengthened. A New York Times poll in September found that 44 percent of Republicans believed the program was “about right,” while 26 percent said that it did not go far enough. Just 23 percent said it went too far.

Most of Mr. Paul’s declared and likely primary rivals have shown a willingness to defer to the intelligence community. Jeb Bush, the former Florida governor, has praised President Obama’s use of the data sweeps, which were started under his brother George W. Bush. And Senator Marco Rubio of Florida denounced the court decision that declared the program illegal. “Now is not the time to end this program,” he wrote in an op-ed article in USA Today.

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The audience at Senator Paul's talk on Saturday at Startup House.CreditJason Henry for The New York Times

Not even Senator Bernie Sanders of Vermont, a self-described socialist who is running for the Democratic nomination, has made N.S.A. reform as big a piece of his campaign as Mr. Paul.

But Mr. Paul sees appeal in making the issue part of a larger critique of the foreign policies of both the Bush and Obama administrations. “Even in the Republican Party, there are a lot of people who are starting to wonder: ‘Are we safer? Is Iran not stronger with Hussein gone?’ ” he said in the interview. “ ‘Are we not more at risk from radical Islam?’ ”

Mr. Paul hopes to sell himself as “a different kind of Republican,” as he put it in San Francisco, and that this will resonate in early-voting states like New Hampshire and Iowa, where his campaign has made younger voters a priority. The 2016 Iowa caucuses will take place while colleges are in session for the first time since 2004.

Mr. Paul’s attention to privacy matters, his campaign is calculating, could show that he is attuned to the concerns of younger, more technologically dependent voters. And it is an important element in a cool factor that he is working hard to cultivate. His website has a store filled with sardonic merchandise — including obsolete computer hardware billed as “Hillary’s hard drive,” wiped clean, and a T-shirt that says “Don’t Drone Me, Bro.”

The venue he chose for his San Francisco appearance, meanwhile — a shared work space called Startup House, where entrepreneurs pay $50 to $450 for desks, Internet access and a stocked refrigerator — was not where one would typically expect to find a Republican running for president. It advertises itself as “a supportive environment for people who aren’t just white guys working on start-ups.” Mr. Paul’s campaign is a new tenant.

The founder of Startup House, Elias Bizannes, said that he had to endure some nasty emails from his members for hosting a Republican, but that Mr. Paul was making strides.

“You’re not going to be the most tech-savvy candidate because you know Snapchat and Instagram,” Mr. Bizannes said. “He definitely gets that.”

A version of this article appears in print on , on Page A15 of the New York edition with the headline: Paul Tries to Stake Territory as Lone Candidate Who’d Guard Civil Liberties. Order Reprints | Today’s Paper | Subscribe